Process of recovering the alkali used in pulp digestion



D. S. II/ICAFEE. PROCESSA OF RECOVEHING THE ALKALI USED IN PULP DIGESTION.

APPLICATION FILED AUG. I6. IBIS.

Patented July 1, 1919.,

mm1/LIQ@ UNITED sTaTns PATENT onnien.

DANIEL SHIELDS MCAFEE, OF NEW YORK, N. Y., ASSIGNOR TO THE BORR COMPANY, A

I CORPORATION OF DELAWARE.

PROCESS OF RECOVERING THE ALKALI USED IN PULP DIGESTION.

Specification of Letters Patent. i

Patented July l, 1919 Application led August 16, 1918. Serial No. 250,125.

To ZZ wlw/n. it may concern lie it known that l. Daxncl. SnncLns Mc- ,l 11i-1n. a citizen ot the llnited States, residing at New York city. in the county otXcw York and State ot New York. have invented certain new and uset'ul Improvements in lrocesses ot' Recovering the Alkali Used in Pulp Digestion: and l do hereby declare the, following to be a tull, clear, and exact description ot' the invention. such as will enable others skilled in the art to which it appertains to make and use the same.

This invention. broadly considered. relates to the manufacture ot' caustic soda t'rom soda ash and lime. More particularly it is applicable to a cyclic process in which the caustic soda used in a digesting operation. and thereby substantially neutralized. is subsequently burned to black ash cinder. the

sodium carbonate content ot' whichis again extracted and treated with lime to l'egenei'- ate the caustic soda.

For example. in the Soda procfss for making paper pulp. where the raw material. such as wood chips. cottonseed hulls, or they like. is treated in suitabledigesters withnthe caustic soda. the non-cellulose mattersoi" the.

raw material arey decomposed by or combine with the caustic, and consequently the alkali is substantially all neutralized during the digesting operation. The. spent` liquor, resultingtrom the digestingoperation, is evaporated to a suitablel consistency, say about ritt@ lie., and the concentrated liquor is then sent to a ealcination furnace, usually of the rotary kiln type, from which a dry soda ash cindei'. generally called black ash cinder. is recovered. This black ash cinder is treated tor the recovery ot' the sodium content thereot. which is recausticized by means of lime to regenerate the caustic soda for subsequent use in the digesters.

'l`he black ash cinder is Composed prineipally ot sodium carbonate, :ind the chemical reaction taking place by the treatment thereof with freshly slaked lime. or unslaked lime and water, is represented by the following equation Nagt(),H-Ca()+HZUIQNaOH-l-Cac() lt has heretoforebeen the gene lal practice, in re-causticizing black ash ci'nder, to leach the cinder by percolation, after it is discharged from thecalcining furnace, and to convey the liquor trom this leaching operation to the lime-slaking units. ()nly a partial reaction between the' sodium ycarbonate and calcium hydroxid takes place in the lime-slaking units. and the mixture is next ,'l`hus. in accordance with this aspect' otI the4 invention. the .sodium carbonate of the soda ash is subjected to the action of freshly slaked lime. or nnslaked lime and a liquid lime-slaking medium. in a closed circuit grinding system. t'rom which the suiiieiently ground product is discharged as a slimeand thev untinished product is returned for a ful'- ther grinding treatment. ln its preferred and complete aspect. however. the present invention inyolves the grinding ot the soda ash and lime. together with a liquid limeslaking n-iedium. in a closed circuit grinding system. The soda ash and lime are preterably t'ed intothis system in a dry condition, and the grinding operation eli'ects. at one and the same time. the slaking ot the lime and the leaching ot the soda ash. The preliminary percolation leaching ot the soda ash. such as heretofore generally practised. is thus completely eliminated. The closed circuit grinding system is preferably composed ot' a grinding mill, such as a tube or ball mill. and a classifier'. so arranged that the discharge t'roin the mill goesto the classifier. from lwhich the finished product is dis` charged while `the unfinished product is returned to the mill.

The accompanying drawing diagrammatically illustrates. by means of a conventional iow sheet, a system of apparatus for producing caustic soda, from 'a spent liquor in which the soda is principally present as carbonate and from lime. This system of apparatus, as illustrated, is designed for carrying out the iu'iproyements of the present invention.V The various'pieees of apparatus of this system are appropriately marked in `the drawing. and the arrangement of the same will be understood from the following description of the use of this system t'or car- `as wood pulp or the like, witli caustic soda,

is effected 1n suitable digesters, and the spent liquor is concentrated to about 36 Be. in suitable evaporators. -The product from the evaporators is conveyed to the incinerators. such, for example, as rotary kilns, and there. lmrned to black ash cinder. The black ash cinder is then conveyed to suitable storage bins. lt will. of course, bc linderstood that this cinder lnay be subjected to a preliminary grinding or breaking operation, if desired.

In accordance. with the improvements of the present invention. the black ash cinder is fed directly from' the storage bin to the closed circuit grinding system. A\ssociated with the soda ash storage bili is a similar storage bin for kiln lime. If desired. the lime may be preliminarily crushed before storage in its bili. and to this end the acco1iipan \'ing drawing illustrates a grizv'xr` and crusher with suitable conveyers and ele vators for effecting such a'prelimiminv treatlnent of the linie. The same or similar preliminary treating devices may. of course. be associated with the soda kash storage bin.

The soda'ash and lime are fed from their respective storage bins by suitable niet hanical feed devices and in the proper proportions to a grinding mill. such` foiexample.'

as a tube or ball mill or like pulverizing device. suitable amount ofl dryv fresh soda ash for making up the losses incidental to the complete process is also fed to the grinding mill from a suitable storage bin therefor. In the grinding mill the soda ash and lime are ground with a suitable liquid limeslaking medium. such` for example. as water, or wash liquor as will be more fully explained hereinafter.

classifier is associated with the'grinding mill so as to form a closed circuit grinding system. Thus, the ground mixture is discharged from the mill into the classifier and the` sufficientlyground product accumulates in a sump and is conveyed in the form of a slime into suitable reaction agitators by means of an elevator, while the insufficiently ground product is returned from the classifier to the grinding mill for further reduction.

The closed circuit grinding system effects a slaking of the lime and a leaching of the sod-a ash cinder, and. to a certain extent. a causticizing reaction between the sodium carbonate and the calcium 'liydi-oxitl, The complete causticizing reaction takes place,- however` in a battery of'reaction agitators. in which the discharged slime from the classifier is agitated in'any suitable manner,

ltion is absent.

as, for example, mechanically, or by air, or both, atv a temperature of about 95o C.

The resulting solution of caustic soda is preferably separated from the residual lime mud (CaCo3) by continuous counter-current. decantation. Thus, in the accompanying drawing, a battery of three thickeners or pulp separators has been illustrated for this purpose. The discharge from the last reaction agitator is conveyed to the first thickener, from which the clear overflow, consisting of a solution of caustic soda. is conveyed to a suitable finished liquor storage container. The residue from thel first thickener is pumped into the top of the second thickener after being mixed with the wash water from the third thickener. The dilute caustic liquor overflowing the second thickener is utilized as the liquid lime-slaking medium in the. grinding mill and for diluting the slime discharged from the classifier into the reaction agitators. This wash water Inay also be iulvantageously used in the classifying operation. The pulp from the, second thickener .is pumped to they third thickener after mixing with fresh wafer. the overflow passing` as previously stated. to the pulp inlet of the second thickener. The washed lime mud from the lastA thickener is pumped or suitably conveyed to storage for eventual reclamation. lt will. of course. be understood that more than three reaction agitators and more than three thickeners may be employed. This number has been illustrated. in the drawing merely by way of example.

From the foregoing description. it will be evident that a most intimate mixture of the reacting agents is secured during the `grinding operation. ln the first place. the lime 1s ground during the slaking operation. so that fresh surfaces are constantly being exposed to the action of the slaking medium. In this manner a nmch more effective slakilig of the lime is attained than is possible by prior methods in which the grinding acln the second place. the grinding of the soda ash cinder constantly exposes fresh surfaces thereof to the action of the solvent liquor for the sodium carbonafe content thereof. whereby a more complete solution of the sodium carbonate value is attained rlhe grinding operation not only assists in effectively slaking the lime` but if also slinics all insolubles so as to protect the tank svstenis from grit.` and disintegrafes and conipletely pulverizes the soda ash cinder. thus facilitating the solution of its sodium earbonate content. The slaked linie also partially reacts with the sodium carbonate of the soda ash during the grinding operation` and because of the greater surface of the finely divided soda ash exposed to the, act ion of the calcium hydroxid and water, a more flfl ltltl lll eective causticizing action takes place than is possible in the lime slaking units heretofore employed.

The resent invention thus enables a substantia ly complete recovery -of the sodium labout 70 to 85% of recoverable sodium car.-

bonate1 so that the increased solids, over priorr practices, to be handled through the thickeners and pumps ,in accordance with the present invention, is practically negligible." The process of the invention enables the provision of a more compact and simple lant than has heretofore been possible, and lurtherfe'nables the complete elimination of oneoperation of the prior practice. The complete cycle of soda recovery-lime sl airing causticizing and washing is made continuons; thus insuring the uniformity of the finished yliquor produced and materially reducing the operating labor required.

In'the slaking of the lime, water is, Aof

l' course, the active lime-slaking agent. While Et is generally preferable to use the Wash liquor lfrom the thickeners for slaklng the lime in the grinding mill, it will, of course, be understood that any other suitable limeslaliing medium may be used, such. for exam le, as fresh Water, a solution oi sodium car ouate in Water, dilute'caust-ic soda, and so on. These and other obvious modifica-- tions of the invention will be understood by those skilled in the art, and it is the intention ofthe appended claims to cover all .such modilications within the, spirit and scope of the invention.

I claim: V i

l. Inv a' cyclic process involving the recausticizing of thehsodium carbonate lcontent of soda ash cindr produced by burning the spentlliquor resultingfrom the use of a caustic soda solution as a diggsting agent, the improvement which consists in feeding unslaked lime and a liquid limeslaking r'.edium into a indin mill and therein bringing the resu ting s aked lime solution into contact with the soda ash cinder to be causticized and vgrinding the mixture, and se aratn theresultilflg solutionof caustic so a fromft ere'sidue; substantially as described.

2.111,9. cyclic'process involving the recausticizi of the sodium'carbonate content of s a `ash cinder produced by burningthe spent liquor resulting from the use ofacaustic soda solution as a digesting agent, the improvement which consists in treating the soda ash cinder With slaked lime in a closed circuit grinding system from whichthe suiliciently ground product is discharged .A and the unfinished product is returned for a further grinding treatment', and separating the resulting solution of caustic soda from the residue of said discharge 1 product; substantially as described. f

3. In a cyclic process involving the recausticizing of the sodium carbonate cons tent of soda ash cinder produced by burning the spent liquor resulting from the use fot a caustic soda solution as a digesting agent,

the improvement which consistsiin subjecting the soda ash cinder to a grinding operation, treatingit during said grinding oper- 4 ation with slaked lime, and separating the resulting solution of caustic soda from the residue; substantiallv as described.

4. In a cyclic process involving the recausticizing ofthe sodium carbonate content of soda ash cinder produced by burning the spent liquor resulting from vthe use of a caustic soda solution as a digesting agent, the improvement which consists in grinding ythe soda ash cinder and lime in conjunction with a liquid lilneeslaking agent and solvent of caustic soda, and separating the resulting solution of caustic soda from the residue; substantially as described.

5. In a cyclic process involving the recausticizing of ,the sodium carbonate content of soda ash cinder produced by burning the spent liquor `resulting from the use of a caustic soda solution as a digesting agent, the improvement which consists in treating the soda ash cinder and lime in conjunction with a liquid lime-slaking medium in a closed circuit. grinding system from which the finished product is discharged and the uninishedproduct is returned for further treatment, and separating the resulting solution of caustic soda from the residue; substantially as described.

6. In a cyclic process involving the recaustcizing of the sodium carbonate contento'f Ysoda ash cinder produced by burning the spent liquor resulting from the usc of, a caustic soda solution as a digesting agent, ythe improvement which consists in gnnding a nnxture of the soda ash clnder caustic'izin of the sodium carbonate content of so ajash cinder reduced by burning the spent li uor resu ting from, the use t of a caustic so a solution as adigesting agent, the nnprovement "which consists in ist3v lution of caustic soda 45 lime-slaking agent soda, and separating the resulting solution' grindinggthesoda ash cinder and lime in conjunction with a liquid lime-.slakin mediumandsolventof caustic soda, disc arging the sumcientlyground product of said t. inding. operation and returning the untent of soda ash cinder produced byA burn` ing the spent liquor resu ting from the use of a caustic' soda solution as a digesting agent, the improvement which consists in y grinding unslaked lime with a li uid lime# slaking agent, bringing the so iurn ash 2o cinder to be causticized into contact there- With during the inding operation, discharging the suciently ground product of said lnding operation and returning the unfinlshed product for a further grinding 2sA treatment, and separating the resulting caustic soda from said discharge product; substantially as .described.

9.` The process of causticizing soda ash which comprises grinding unslaked lime 3e 'with a liquid lime-slaking agent, and bringing the' sodium carbonate of the soda 4ash' to be. causticized into contact therewith during said grinding operation; substantially as described.

d10. The process of causticizing soda ash which comprises subjecting the soda ash to a'grin'ding operation, treating the soda ash 'during Asald grinding operation with slaked lime, and separatingthe resultingsolution 4e of caustic soda from the residue; substantially as described.

11. The process of causticizing soda ash" with lime which comprises grinding soda ash and lime 1n conjunction lWith a liquid and solventof caustic of caustic soda from thejresidue; substan tially as described. 12. The process of ycauticizing soda ash with lime which comprises treatingsoda ash and lime in conjunction with a liquid lime-slakingv medium,l in a closed circuit` grinding system from which the finishedy product is dischar ed and the unfinishedy product is return for further treatment,

'ther product; substantiallfy ja residue is returned for a further and separating the lresulting solution of caustic soda from the residue; substantially as described. p

' 13. The process of producing caustic soda from soda ash land lime which comprises. grinding ash and lime and slaking the lime during the vgrinding operation, agitating the ground mixture with a solvent of caustic soda, and separating the resulting solution of caustic soda from the residue; substantiallyas .described.

15. The process of causticizing soda ash with lime which comprises grinding soda `ash and lime -,in conjunction with a liquid lime-slaking medium and solvent of caustic soda, discharging the suiiiciently ground product of said grinding o eration and returning the unfinished pr uct thereof for a further grinding treatment agitating said discharged product to com lete the causticizing of sind soda ash, an separating the resulting solution yof caustic soda from the residue; substantially as described.

16. The process of causticizin i soda ash which comprises grinding uns aked lime with a liquid lime-slaking agent, bringing the sodium carbonate to be 'causticized into contact therewith during the Higrinding operation, discharging the su product of said grinding operation and returning the uninishedivproduct for a turlding treatment and separating the resulting caustic soda from said discharge as described.

17. The process o which comprises grindingr soda as in conjunction with a iquid lim medium and solvent of caustic soda Vsubjecting the ground product to a classi yin operation from which slime is discharge and grinding arating the resultant cans slime; substantially as deand lime -slaking operation, and se tlc soda from sai scri In testimony whereof I aix my signature.

DANIEL sHmLDs MCAFEE.

causticizin soda ash ciently ground 

